1. Field of the Invention
The present invention relates to a thin film transistor and method of fabricating the same and, more particularly, to a thin film transistor and method of fabricating the same, in which an amorphous silicon layer is formed on a substrate, a capping layer containing a metal catalyst having a different concentration according to its thickness is formed on the amorphous silicon layer, the capping layer is patterned to form a capping layer pattern, and the amorphous silicon layer is crystallized, such that the density and position of seeds formed at an interface between the amorphous silicon layer and the capping layer pattern is controlled, thereby improving the size and uniformity of grains, and in which polycrystalline silicon of desired size and uniformity is selectively formed at a desired position by one crystallization process, resulting in a thin film transistor having excellent and desired properties.
2. Description of Related Art
Generally, a polycrystalline silicon layer is being widely used as a semiconductor layer for a thin film transistor because the polycrystalline silicon layer has high field effect mobility, is applicable to a high-speed operation circuit and may implement a CMOS circuit configuration. The thin film transistor with the polycrystalline silicon layer is primarily used for an active device of an active matrix liquid crystal display (AMLCD), a switching device and a driving device of a flat panel display device such as an organic light emitting display (OLED).
The polycrystalline silicon layer for use in the thin film transistor is typically fabricated by a technique using high-temperature annealing, a laser annealing method, or the like after an amorphous silicon layer is deposited. In the laser annealing method, a low-temperature process is possible and high field effect mobility is achieved, but expensive laser equipment is required. Accordingly, alternative techniques have been extensively studied.
Presently, a method of crystallizing amorphous silicon using metal catalyst is being extensively studied because of its advantage of fast crystallization at a lower temperature compared to a solid phase crystallization method annealing for long-time at a high temperature for the crystallization. The crystallization method using the metal catalyst may be classified into a metal induced crystallization method and a metal induced lateral crystallization method. However, there is a problem with the methods using the metal catalyst that leakage current increases due to metal contamination, thereby deteriorating the properties of a thin film transistor.
Meanwhile, there are a technique of forming a good quality of polycrystalline silicon layer by adjusting the concentration of metal ions using an ion implanter and applying high-temperature treatment, rapid annealing, or laser irradiation in order to form a good quality polycrystalline silicon layer having a reduced amount of the metal catalyst, and a metal induced crystallization method of blending an organic layer having viscosity and a metal catalyst in a liquid state in order to planarize the surface of a polycrystalline silicon layer, and of depositing a thin film using a spin coating method and then crystallizing the thin film using an annealing process. However, even this crystallization method has a problem in obtaining a large sized and uniform grain that is the most important feature in the polycrystalline silicon layer.
A method of fabricating a polycrystalline silicon layer by a crystallization method using a capping layer was developed to solve the aforementioned problem, as described in Korean Laid-open Patent No. 2003-0060403. In this method, a metal catalyst layer is formed on a substrate, a capping layer is formed on the metal catalyst layer, and then an amorphous silicon layer is formed on the capping layer. The metal catalyst is diffused into the amorphous silicon layer through the capping layer by annealing or laser to form seeds, and a polycrystalline silicon layer is obtained using the seeds. The method has an advantage capable of preventing metal contamination because the diffusion of the metal catalyst is made through the capping layer.
However, with this crystallization method, it is difficult to control the concentration of the metal catalyst uniformly and to control crystallization position and grain size. In particular, it is difficult to pattern the metal catalyst layer uniformly since the metal catalyst may be not protected when the metal catalyst layer is patterned.